1. Field of the Invention
This invention relates to a method for the treatment of metallic surfaces for the protection of the surface from oxidation and for enhancing its solderability and, more particularly, to treating electronic components such as printed wiring boards (PWB""s) containing both copper and gold circuitry to provide a protective coating on the copper before the copper is soldered with the gold features on the substrate not being discolored or otherwise electrically affected by the treatment.
2. Description of Related Art
In the manufacture of metal containing components which are to be soldered, it is necessary to protect the metal from oxidation to enhance its solderability. For convenience the following description will be directed to the manufacture of printed wiring boards (PWB""s) or other electronic components having copper circuitry which is to be soldered and where it is essential that the copper not be oxidized or otherwise corroded prior to soldering.
Typically, many such electronic components having copper circuitry thereon also have gold leads or other electrical connections which are generally used to connect the electronic component to another electronic component. Currently however, processes used to protect the copper circuitry prior to soldering deleteriously affect the gold metallurgy on the substrate by staining the gold or otherwise electrically affecting the gold.
The processes used today for protecting copper prior to soldering employ a protective coating deposited on the copper. This coating is used as an alternative to hot air solder leveling (HASL) and other metallic printed circuit board surface finishes. The coating provides protection against copper solderability degradation caused by various process steps in the fabrication process such as exposure to multiple heat cycles during electronic component fabrication for surface mount technology (SMT) and mixed technology PWB assembly.
Generally, copper protective coating systems utilize a number of steps including cleaning, microetching and acid rinsing followed by the formation of a protective coating on the copper using a solution containing a protective forming agent. The protective forming agent is typically an imidazole or benzimidazole derivative and the coating is generally termed an organic solderability preservative (OSP) coating.
A number of patents have issued in this area which have attempted to solve the problems associated with providing a protective coating on copper circuitry.
U.S. Pat. No. 5,658,611 to Ishiko et al. provides an aqueous surface protection composition for PWB""s containing a benzimidazole derivative and adjusted to a pH of 1-5 with a salt forming acid of a heavy metal such as copper, manganese and zinc in an amount not higher than 50 ppm.
In U.S. Pat. No. 5,173,130 to Kinoshita et al. a process is disclosed for the surface treatment of copper which comprises immersing the surface of the copper in an aqueous solution containing a benzimidazole compound having an alkyl group of at least three carbon atoms at the 2-position and an organic acid. Similarly, in U.S. Pat. Nos. 5,498,301 and 5,560,785 to Hirao et al. a water-based surface treatment agent used to protect copper on a printed wiring board with excellent heat-resistance and moisture-resistance utilizes as an active ingredient a 2-arylimidazole compound.
In EPA Publication No. EP 0 791 671 to Hirao et al. a surface treating agent for copper is disclosed comprising an aqueous solution containing an imidazole compound or a benzimidazole compound, a complexer and iron ions. It is contended that the surface treating agent forms a chemical film selectively on the surface of copper while forming no film on other metals.
In U.S. Pat. No. 5,362,334 to Adams et al. a composition and process for the surface treatment of metallic surfaces such as copper circuitry on printed circuit boards is disclosed which comprises treating the surface with an aqueous solution comprising a benzimidazole compound having either a halogenated phenyl group, a halogenated benzyl group or a halogenated ethyl phenyl group in the 2-position. In U.S. Pat. No. 5,376,189 to Kukanskis a composition and process are disclosed for the treatment of metallic surfaces such as copper on printed circuit boards which comprises treating the surface with an aqueous solution comprising a benzimidazole compound which has at least one carboxylic or sulfonic acid group directly or indirectly attached to the benzimidazole compound.
Unfortunately, current processes for producing a protective coating on the copper surface still, in varying degrees, deleteriously affect the gold metallurgy on the electronic substrate because it attracts the coating used to coat the copper and causes cosmetic defects in the form of staining and may reduce the conductivity of the gold contact surface.
Bearing in mind the problems and deficiencies of the prior art, it is therefore an object of the present invention to provide an improved method to selectively deposit an organic solderability preservative (OSP) coating on copper surfaces of electronic components such as printed wiring boards to serve as a protective coating to preserve the solderability of the copper surfaces while substantially excluding gold surfaces from attracting the same protective coating and staining the gold and/or affecting the conductivity of the gold.
It is another object of the present invention to provide electronic components such as printed wiring boards made using the method of the invention.
Other advantages of the invention would be readily apparent from the following description.
In one aspect of the invention the method for the treatment of metallic surfaces to protect the surface from oxidation and for enhancing its solderability and, in particular, to treating electronic components such as printed wiring boards containing both copper and gold circuitry to provide a protective coating on the copper before the copper is soldered with the gold features on the substrate not being discolored or otherwise electrically affected by the treatment has been improved. In the method for the treatment of metallic surfaces for protection of the surface from oxidation for enhancing its solderability the metallic surface is cleaned, optionally etched and then contacted with a solution of an organic solderability preservative material typically a triazole, imidazole or benzimidazole or derivatives thereof whereby an organic solderability preservative protective film or coating is formed on the metallic surface, the improvement comprising:
contacting the metallic surface with a pre-treatment solution prior to contact of the metallic surface with an organic solderability preservative material typically a solution of triazole, imidazole or benzimidazole or derivatives thereof used to provide the organic solderability protective coating, the pre-treatment solution comprising a benzimidazole compound having the formula: 
wherein R1, R2, R3, and R4 are independently selected from the group consisting of hydrogen atoms, substituted or unsubstituted alkyl groups of 1-10 carbon atoms, preferably a lower alkyl of 1-4 carbon atoms, substituted or unsubstituted aryl groups, halogen atoms, alkoxy groups of 1 to 10 carbon atoms, preferably 1 to 4 carbon atoms, alkyl amino groups of 1 to 10 carbon atoms, preferably 1-4 carbon atoms, cyano groups, nitro groups and mixtures thereof.
In a preferred aspect of the invention R1, R2, R3, and R4 are all hydrogen.
The benzimidazole compound used to form the pre-treatment solution is dissolved to form a solution, preferably aqueous, and preferably includes a corrosion inhibitor compound such as an alkanolamine, e.g., a C1-C4 alkanolamine such as triisopropanolamine, and a buffering agent such as a salt like ammonium acetate to stabilize the pH. The solution may be used over a wide range of temperatures typically 70xc2x0 F. to the boiling point of the solution, typically 70xc2x0 F. to 120xc2x0 F., preferably 70xc2x0 F. to 90xc2x0 F. The pH of the solution may also range widely from about 1 to 11, and is preferably about 7-10, e.g., 8-9.
In another aspect of the invention, electronic components made using the method of the invention are provided.
The present invention for convenience is specifically described for a protective coating system for copper metallurgy on printed wiring boards (PWB""s) including copper pads and through holes. The protective coating is used as an alternate to hot air solder leveling (HASL) and other metallic PWB surface finishes. The coating of the invention provides excellent protection against copper solderability degradation caused by exposure to multiple heat cycles during SMT and mixed technology PWB assembly.
The above referenced patents are directed to the surface protection of copper metallurgy on PWBs and disclose the basic OSP process for treating the PWB. In general, the surface of the copper or copper alloy is first cleaned by immersion of the board in a cleaner and then preferably etched to increase the adhesion of the copper. The copper is then immersed in a solution containing an organic solderability preservative material such as triazole, imidazole, benzimidazole or derivatives thereof. Spraying or other forms of coating may also be used.
Cleaning of the PWB may be performed using any number of cleaners including alkaline, acid or neutral cleaners and is preferably an acidic cleaner like sulfuric acid, HCl and citric acid. The PWB is typically contacted with the cleaner for up to about 5 minutes at a temperature of 80xc2x0 to 120xc2x0 F.
All the steps in the conventional OSP process are followed by rinsing the substrate typically with deionized water. Rinsing is usually performed at an ambient temperature, e.g., 75xc2x0 F.
An etchant is preferably used to microetch the surface of the copper to increase the adhesion of the copper and an etchant such as a persulfate salt such as sodium persulfate or sulfuric peroxide type material at 70xc2x0-100xc2x0 F. for up to about 1 min. is typically employed.
It is also preferable, but optional, to use a dilute sulfuric acid solution such as a 5% by weight sulfuric acid solution for up to about 2 minutes at 70xc2x0 to 90xc2x0 F. to prevent drag-down contamination of the OSP solution by the etchant solution.
Following the conventional procedure, the substrate is now ready to be treated with the OSP solution to form the protective layer on the copper metallurgy. This is usually performed at a temperature of 95xc2x0 to 115xc2x0 F., preferably about 105xc2x0 to 110xc2x0 F. for about 15 seconds to 1 minute.
After the OSP treatment, the substrate is rinsed and typically dried by dry forced air. The substrate is now ready to be soldered and will be protected against oxidation until the soldering step is performed.
As can be seen from the above cited patents, a number of OSP solutions have been developed which may be used to form the OSP coating. The present invention treats the PWB before the OSP treatment to precoat the copper and any suitable OSP solutions in the prior art may be used to provide the final coating on the substrate metallurgy.
For example, U.S. Pat. No. 5,362,334, supra, OSP solutions containing the following compound may be employed: 
wherein at least 1 of the constituents, R1, R2, R3, R4, and R5 must be selected from the group consisting of halogens, substituted or unsubstituted halogenated alkyl groups and substituted or unsubstituted halogenated aryl groups; and
wherein the remaining constituents, R1, R2, R3, R4, and R5, are selected from the group of consisting of hydrogen, nitro groups, substituted or unsubstituted alkyl groups and substituted or unsubstituted aryl groups.
In U.S. Pat. No. 5,376,189 compounds represented by the following general formula are used in the OSP solution: 
wherein at least 1 of the constituents R1, R2, R3, R4, and R5, must be selected from the group consisting of carboxylic acid groups, sulfonic acid groups, alkyl groups substituted with carboxylic or sulfonic acid groups and aryl groups substituted with carboxylic or sulfonic acid groups.
In U.S. Pat. No. 5,658,611, a benzimidazole derivative which is used in OSP solutions contains an effective component represented by the general formula of: 
wherein X is a radical, with the number of carbon atoms from 1 to 7, of the group consisting of alkyl group, halogen atoms amino group, lower dialkylamino grup, lower alkoxyl group, cyano group, acetyl group, benzoyl group, carbamoyl group, formyl group, carboxyl group, lower alkoxylcarbonyl group and nitro group, n and p are integers of 0 to 4, and m is an integer of 1 to 10.
The solution is adjusted to pH range from 1 to 5 with a salt-forming acid and the aqueous solution has a metal content of a heavy metal selected from the group consisting of copper, manganese and zinc and which metal is not higher than 50 ppm.
In U.S. Pat. Nos. 5,498,301 and 5,560,785 a water based surface treatment agent contains as an active ingredient a 2-arylimidazole component represented by the following formula: 
wherein R is a hydrogen atom or a methyl group, R1and R2 are hydrogen atoms, lower alkyl groups or halogen atoms, and R3 and R4 are hydrogen atoms, lower alkyl groups, halogen atoms, lower alkoxyl groups, di-lower alkylamino groups, cyano groups or nitro groups.
In U.S. Pat. No. 5,173,130 OSP solutions contain compounds represented by the following general formula are used: 
wherein R1 represents an alkyl group having at least 3 carbon atoms.
Furthermore, compounds represented by the following general formula can also be used for carrying out the process: 
wherein R1 represents an alkyl group having at least 3 carbon atoms, and R2, R3, and R4 represent a lower alkyl group, a halogen atom, a nitro group or a hydrogen atom, with the proviso that at least one of R2, R3, and R4 represents a lower alkyl group, a halogen atom or a nitro group.
In EP 0 551 112 a benzimidazole derivative used in OSP solutions is represented by the following formula: 
wherein R1 is a hydrogen atom, a lower alkyl group, or a halogen atom; R2 is a hydrogen atom or a lower alkyl group; R3 is an alkylene group with 1 to 18 carbon atoms; and n and m are integers from 0 to 3.
In EP 0 791 671 an OSP solution containing an imidazole compound or a benzimidazole compound is shown which includes a complexor and iron ions.
In the present invention, the above OSP process is improved by using a pre-treatment step wherein the substrate is contacted with a pre-treatment solution before the OSP treatment, the pre-treatment solution comprising a benzimidazole compound having the formula: 
wherein R1, R2, R3, and R4 are independently selected from the group consisting of hydrogen atoms, substituted or unsubstituted alkyl groups of 1-10 carbon atoms, preferably a lower alkyl of 1-4 carbon atoms, substituted or unsubstituted aryl groups, halogen atoms, alkoxy groups of 1 to 10 carbon atoms, preferably 1 to 4 carbon atoms, alkyl amino groups of 1 to 10 carbon atoms, preferably 1-4 atoms, cyano groups, nitro groups and mixtures thereof.
Using the process of the invention it has been found that the pre-treatment step enhances the final protective coating on the copper metallurgy with other metallurgy such as gold on the substrate being substantially unaffected by the OSP treatment process used to protect the copper surface.
The pre-treatment solution is preferably an aqueous solution having a pH of about 1 to 11, preferably 7 to 10 and most preferably 8-9 and is used at a temperature of about 70xc2x0 to 120xc2x0 F. or higher, preferably 70xc2x0 to 90xc2x0 F. The benzimidazole active ingredient in the solution is typically used at a range of about 0.1 to 10 g/l, preferably 0.5 to 2 g/l, e.g., 1 g/l. The contact time may also vary widely from about 5 seconds to 10 minutes, preferably 15 seconds to 1 minute, typically 15 sec. for a conveyor system and 1 minute for a dip tank process.
Solubilizers and buffering agents are typically used to dissolve the benzimidazole active ingredient in the solution and maintain the solution pH, respectively. An alcohol such as isopropanol may be used to solubilize the active ingredient wherein the active ingredient is dissolved in the alcohol and then added to the water used to make the pretreating solution. If isopropyl alcohol is used, it has been found that the alcohol is typically vaporized from the working solution used to pretreat the substrate. Heat may also be used to solubilize the active ingredient. Corrosion inhibitors such as triisopropanolamine, and other alkanolamines (e.g. C1-C4 alkanolamine) and other such materials are preferably used in the composition in an amount of about 0.05 to 5 g/l, preferably 0.5 to 1 g/l, e.g., 0.7 g/l. Salts, preferably ammonium salts such as ammonium acetate, ammonium formate and the like may be used as a buffering agent to maintain the solution pH in an amount of about 0.1 to 5 g/l, preferably 0.25 to 0.6 g/l, e.g., 0.4 g/l.
It will be understood by those skilled in the art, that higher contact temperatures will usually decrease the amount of the time needed to provide the desired pre-treatment efect. Similarly, increased concentration will typically decrease the amount of time needed to provide the desired pre-treatment effect.